The central hypothesis to be pursued in this project is that the presentation of specific antigens by B cells occurs in a specialized subcellular compartment(s). Preliminary studies have identified two candidate compartments termed Class II vesicles (CIIV) and multilaminar Class II vesicles (MCV). CIIV contain both internalized B-cell antigen receptors (BCR) and Class II molecules, suggesting that a fraction of internalized antigen-BCR complexes may be diverted from the bulk endocytic pathway into CIIV, where processing and loading of peptide fragments onto Class II molecules occurs. MCV are a distinct intracellular compartment that also contains Class II molecules, as well as an unidentified 46 kDa marker protein. This project will use biochemically purified preparations of CIIV and MCV to clarify the roles of these specialized compartments in the processing of BCR-associated antigens. In addition, various BCR mutants will be used to identify the sequence determinants responsible for the sorting of antigen-bearing BCR into CIIV and MCV.